The present invention relates to a polishing system capable of improving uniformity of a surface of a flat object to be polished, for example, a semiconductor substrate such as a silicon wafer, a polishing method using the polishing system, a polishing pad provided on the polishing system, and a method of forming the polishing pad.
In a process of forming LSIs, planarization for films such as an interlayer insulating film is very important.
Various means for planarizing films have been proposed, and in recent years, attention has been given to a CMP (Chemical-Mechanical Polishing) process utilizing mirror-polishing for silicon wafers, and methods for planarizing films by making use of such a CMP process have been developed.
In a related art method for planarizing a wafer utilizing the CMP process, polishing is performed in a state in which a polishing pad is pressed on a surface of a wafer by a drive means such as an air cylinder.
In the related art method, however, since loss in the mechanical transmission of a drive means and non-uniformity of a pressing force have not been examined, there has arisen non-uniformity in an effective polishing pressure applied from a polishing pad to a wafer due to both loss in the mechanical transmission of a drive means and uniformity in the pressing force. Such non-uniformity in the polishing pressure has degraded uniformity of a polished surface of a wafer after polishing. In addition, uniformity of a surface of a wafer is defined based on a variation in a residual amount over the entire surface of the wafer.
On the other hand, an effective area of a polishing surface of a polishing pad varies depending on irregularities on a surface of a wafer, so that a projecting portion on the surface of the wafer is polished in a large amount and a recessed portion in the surface of the wafer is polished in a small amount. This degrades uniformity of the polished surface of the wafer.
Further, inhomogeneity of a polishing surface of a polishing pad, which is transferred on a surface of a wafer, is one factor for degrading uniformity of a polished surface of the wafer.
A distribution of a slurry as a polishing material supplied between a wafer and a polishing pad upon polishing differs depending on a position to which the slurry is supplied or differs between inner and outer peripheral sides of the polishing pad, and non-uniformity of the distribution of the slurry degrades uniformity of the wafer surface.
It is essentially difficult to remove irregularities on a wafer surface, inhomogeneity of a polishing surface of a polishing pad, and non-uniformity of a slurry.
Incidentally, in accordance with a PRESTON's equation, an amount removed by polishing is proportional to a polishing pressure, a relative velocity between a polishing pad and an object to be polished, and a working time.
Accordingly, it may be considered that even if there exist irregularities on a wafer surface, inhomogeneity of a polishing surface of a polishing pad, and non-uniformity of a slurry, it is possible to improve uniformity of a wafer surface by positively adjusting a polishing pressure during polishing.
The present invention is intended to provide a polishing system for polishing a surface to be polished of an object to be polished by a polishing pad, which is capable of positively adjusting a polishing pressure, and improving uniformity of the surface to be polished of the object to be polished even if there exist factors degrading uniformity of the polished surface of the object to be polished, and a polishing method using the polishing system.
Further, the related art method of planarizing a wafer utilizing the CPM process is based on a lapping technique. In such a method, an area of a polishing pad is larger than that of a wafer and the polishing pad is applied on the entire surface of the wafer at a time, and also the polishing pad is rotated at a low velocity. This configuration causes problems in terms of accuracy, such as flatness of a wafer surface, uniformity in the polished amount within a wafer surface, and instability of the polishing rate expressed by a polished amount per unit time, and also causes a problem in terms of low throughput.
Specifically, when a polishing pad is applied to the entire surface of a wafer at a time, a high-level region on the wafer surface is polished in a large amount and a low-level region is polished in a small amount. Also, since a peripheral velocity of a polishing pad differs between inner and outer peripheral sides, a polished amount becomes smaller on the inner peripheral side and becomes larger on the outer peripheral side. Further, an amount of a slurry as a polishing material supplied between a polishing pad and a wafer upon polishing differs between inner and outer peripheral sides of the polishing pad.
Accordingly, when there exist non-uniformity in the polished amount and instability of the polishing rate within a wafer surface due to the above causes, it is difficult to perform accurate polishing by numerical control of the polishing system.
Meanwhile, if a size of a polishing pad is made small relative to a wafer size, a slide-contact distance between a wafer and the polishing pad per unit polishing pad area becomes larger, leading to severe wear of the polishing pad. This frequently requires exchange of the polishing pad. Further, there arises an inconvenience that a polishing pad is liable to be clogged.
Incidentally, to make finer an interconnection pattern and the like, it is required to enhance an accuracy of the exposure for preparation of the interconnection pattern and the like. This requires a technique of further enhancing the flatness of a polished surface of a semiconductor wafer. Even in the case of increasing a diameter of a semiconductor wafer, it is similarly required to further enhance the flatness of a polished surface of a semiconductor wafer.
As a result of examining polishing in view of the foregoing, it becomes apparent that any polishing method using the related art polishing system cannot ensure a highly accurate flatness required for a fabrication process after the 0.25 .mu.m's generation.